Update YamlIO documentation for ScalarTraits

[arphaman]

Summary

This patch updates the documentation for ScalarTraits by adding the missing context parameter
to the output and input method.

llvm/trunk/docs/YamlIO.rst

 
 .. code-block:: c++
 
     using llvm::yaml::ScalarTraits;
     using llvm::yaml::IO;
 
     template <>
     struct ScalarTraits<MyCustomType> {
-      static void output(const T &value, llvm::raw_ostream &out) {
+      static void output(const T &value, void*, llvm::raw_ostream &out) {
         out << value;  // do custom formatting here
       }
-      static StringRef input(StringRef scalar, T &value) {
+      static StringRef input(StringRef scalar, void*, T &value) {
         // do custom parsing here.  Return the empty string on success,
         // or an error message on failure.
         return StringRef();
       }
       // Determine if this scalar needs quotes.
       static bool mustQuote(StringRef) { return true; }
     };